Graphical displays serve a number of important functions. For example, graphical displays may record and store data compactly. Graphical displays are also strong and convenient tools for data analysis. Unlike numerical data analytic procedures (e.g. data reduction techniques), graphical display methods show data in a way that allows users to recognize general behavior, to study details and to find the unexpected. Additionally, graphical displays are relevant for effective data communication in presenting qualitative information to others.
The importance of graphical display techniques in data analysis and communication is well recognized. In particular, aspects of graphical perception (the ability of the human eye-brain system to decode quantitative information from graphical displays, to summarize information and to focus on salient features) have been explored. See, e.g., John M. Chambers et al., Graphical Methods for Data Analysis, Duxbury Press, Boston (1983); William S. Cleveland, The Elements of Graphing Data, Wadsworth Advanced Book Program, Monterey (1985); Edward R. Tufte, The Visual Display of Quantitative Information, Graphics Press, Cheshire, CT (1983); William S. Cleveland, Visualizing Data, Hobart Press, Summit, NJ (1993). Similarly, technological advances in the computer peripheral and printer industries as well as advances in computer languages have made graphical displays easier to generate. See, e.g., Richard A. Becker et al., The New S Language, Wadsworth & Brooks/Cole, Pacific Grove, CA (1988).
Although the above references discuss some general graphical techniques and aspects of graphical perception, effective graphical display techniques are not yet available for many data situations. In particular, effective methods for comparing large amounts of data from multiple time series (i.e., data obtained as a function of time) in a space-efficient manner have proven difficult. Thus, there is a need for an effective method of graphically displaying time series data that permits individual values to be seen, highlights short-term fluctuations and is capable of showing large amounts of data in a relatively small amount of space.